A data network comprises a number of source nodes, each source node receiving traffic from numerous traffic sources, and a number of sink nodes, each sink node delivering data to numerous traffic sinks. The source nodes can be connected to the sink nodes directly or through core nodes. Quite often, source nodes and sink nodes are paired so that a source node and its associated sink node are included within an edge node. The capacity of a given data network is determined by the capacities of the edge nodes and the core nodes.
Each link between nodes (source node to core node or core node to sink node) may comprise multiple channels. An optical multi-channel link uses Wavelength Division Multiplexing (WDM). WDM allows a given optical link to be divided into multiple channels, where a distinct stream of data may be transmitted on each channel and a different wavelength of light is used as a carrier wave to carry each of the multiple channels within the optical link.
A core node that connects source nodes to sink nodes using multi-channel links may be required to switch as much as an entire incoming link to an outgoing link or as little as an incoming channel to an outgoing channel. The latter is called channel switching.
The capacity of a path from any source node, through a core node, to any sink node can be dynamically modified. Network control (modification, or reconfiguration, of path capacity) is preferably exercised primarily at the edge nodes. That is, an edge node determines that a given path should be reconfigured, determines the nature of that reconfiguration and sends a configuration update to a core node in the given path. Accordingly, a simple network core structure is essential for realizing a high-capacity, high-performance network in which the core node connectivity can be adapted according to spatial traffic variation.
In an agile network, time coordination between the edge nodes and the core nodes is required to enable adaptive reconfiguration. That is to say, if an edge node determines that a given path should be reconfigured and indicates the required reconfiguration to a core node, time coordination (a) allows the core node to specify a time at which the reconfiguration will take place and (b) allows the respective edge nodes to react appropriately. Without time coordination, a large guard time, which can be of the order of several milliseconds, would be needed between successive configuration updates. A network having bufferless channel-switching core nodes and using a time-locking (time-coordination) technique is described in the applicant's U.S. patent application Ser. No. 09/286,431, filed on Apr. 6, 1999, and titled “Self-Configuring Distributed Switch”, the contents of which are incorporated herein by reference. The connectivity of each core node is modified in response to traffic-load variations as reported to each core node controller by the edge nodes. To enable frequent reconfiguration without an excessive guard time, each edge node must be time-locked to each core node.
The number of sink nodes that a given source node can reach directly is referred to herein as the topological reach of the source node. Coarse switching, such as link switching or channel switching (described hereinafter), limits the topological reach of a source node and may necessitate tandem switching (described hereinafter) for data streams of low traffic intensity. A data stream is defined as data that is transferred from a source node to a sink node via a particular path (i.e., via a particular core node). A single fiber link may support numerous wavelength channels. Tandem switching may even be required with individual channel switching because the number of channels emanating from a source node would typically be smaller than the number of sink nodes addressable by the source node.
Tandem switching requires that a source node, wishing to send data to a destination sink node that is not reachable directly through a given core node to which the source node is connected, send the data, through the given core node, to an intermediate sink node. From the source node associated with the intermediate sink node, the destination sink node is reachable through the same or another core node.
A time-division-multiplexing-based bufferless-core network is described in the applicant's U.S. patent application Ser. No. 09/550,489, filed on Apr. 17, 2000, and titled “High-Capacity WDM-TDM Packet Switch”, the contents of which are incorporated herein by reference. In a time division multiplexed (TDM) communications system, each channel is divided into time slots and the time slots are arranged in frames of a predetermined number of time slots. Each time slot is associated with a spatial attribute that corresponds to a destination. The use of TDM can significantly increase the topological reach and eliminate the need for tandem switching. The network may be frequently reconfigured by altering the spatial attributes of a subset of the time slots. Implementing TDM requires fast switching core nodes.